A bicycle is normally provided with a rear derailleur active on a sprocket assembly, which consists of a series of coaxial toothed wheels (sprockets), having different diameters and numbers of teeth, coupled with the hub of the rear wheel.
A bicycle can furthermore also be provided with a front derailleur associated with a crankset, which consists of a series of toothed wheels (crown gears) having different diameters and numbers of teeth associated with a pin of the bottom bracket set in rotation by a pair of pedals.
Usually, the crown gears of the crankset are two or three in number having increasing diameters from the crown gear closest to the bicycle frame.
The derailleur, whether front or rear, engages a transmission chain extending in a closed loop between the sprocket assembly and the crankset, which is displaced onto toothed wheels of different diameters and numbers of teeth, so as to obtain different gear ratios.
In particular, downward gearshifting is said when the chain passes from a toothed wheel of larger diameter to a toothed wheel of smaller diameter, whereas upward gearshifting is said when the chain moves from a toothed wheel of smaller diameter to a toothed wheel of larger diameter. Concerning this, it should be noted that with reference to the front derailleur, downward gearshifting corresponds to the passage to a lower gear ratio and upward gearshifting corresponds to the passage to a higher gear ratio.
The displacement in the two directions of the front derailleur is obtained through an actuation device mounted on the handlebars so as to be easily maneuvered by the cyclist.
More in particular, in a mechanical gearshift, the front derailleur is moved between the crown gears of the crankset, in a first direction (upwards) by a traction action exerted by a normally-sheathed inextensible control cable (commonly called Bowden cable), in a second opposite direction (downward) by the release of the traction of the cable and by the elastic return action of a spring provided in the derailleur itself.
The front derailleur is actuated by a linkage that actuates a chain guide adapted for physically displacing the chain between different engagement positions on the crown gears.
The chain guide comprises an inner plate and an outer plate substantially parallel to one another and substantially parallel to the planes on which the crown gears lie. The inner plate is spaced from the outer plate and both are simultaneously and rigidly translated by the traction or release of the control cable.
During downward gearshifting, the outer plate of the derailleur intercepts the chain and derails it from the crown gear engaged making it fall onto a smaller crown gear arranged closer to the frame.
During upward gearshifting, the inner plate of the derailleur intercepts the chain and derails it, accompanying it, towards a larger crown gear arranged farther from the frame.
In the actuation device, the control cable is actuated in traction or in release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is actuated by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In any case, the actuation device must provide for the cable-winding bush to be held still in rotation in a number of predetermined angular positions, angularly spaced apart by predetermined indexing angles. This function is obtained with the so-called indexers, many types of which are known in the prior art, variously active between the cable-winding bush and the fixed casing of the device.
Usually, the indexer comprises more than one stop position to position the front derailleur at the smallest crown gear of the crankset, as illustrated for example in document U.S. Pat. No. 8,485,060 and in document U.S. Pat. No. 7,285,064.
The cyclist, therefore, actuates the control lever to engage the most appropriate stop position as a function of the toothed wheel of the cassette engaged by the chain when the latter is on the smallest crown gear of the crankset.
The Applicant has perceived that it is essential to ensure a correct passage of the transmission chain between the largest crown gear and the smallest crown gear during downward gearshifting from the largest crown gear of the crankset.